Finding Top Dead Centre

It's said 'Timing is Everything' and in the case of internal combustion engines it most certainly is. Tiger Cub and Terrier machines don't have any timing marks, so how can you determine just where top dead centre is in order to accurately set the ignition timing?

 

Here is a reliable static method that works a treat, although you will need a few items to help.

A 'piston stopper' (shown below) made from an old spark plug. The central porcelain/insulator section has been removed by filing off the retaining ridge that holds it in place enabling the centre to be pulled out.

Once the centre has been removed, use either a suitably sized bolt or a rod that will fit tightly in its place. One that will extend 2 - 3 inches down from the end of the spark plug is ideal. The piston stopper in the picture is one I use and which the centre rod can be positioned at different points if required, although in this instance it was fixed in place.

The timing disc (shown at the top of the page) is located on the end of the crankshaft to indicate top dead centre, bottom dead centre and all degrees in between.

You'll also need a timing pointer. This is one I made for my own Tiger Cub (it has two pointers, the reason for the second one will be explained later). It's bolted to a fixed point on the engine to be used in conjunction with the degree disc to indicate crankshaft position.

How to find top dead centre

Remove the primary drive side cover to reveal the rotor on the end of the crankshaft. Remove the valve covers. Put the engine in 4th gear so that the crankshaft can be turned over easily by moving the rear wheel. Remove the spark plug and by turning the rear wheel forward, rotate the engine until the inlet valve closes and the piston begins to ascend on the compression stroke.

Screw the piston stopper into the spark plug hole and continue rotating the rear wheel slowly until the piston comes to a halt at the 'stopper'.

Fit the degree disc to the end of the crank (I used a small powerful horseshoe magnet to secure it to the rotor bolt) and fix the timing pointer in a position that will indicate the position of the disc.

 

Set the degree disc to 0 TDC at the pointer then by using the rear wheel, rotate the engine backwards so that the piston descends past bottom dead centre and starts ascending again until it once again comes up against the 'stopper'. Note the reading on the degree disc - it was a total of 290 degrees from 0 TDC on my Cub - TDC to BDC = 180 + another 110 = 290.

Now halve the total number to give 145 and move the engine forward once more until it has rotated 145 degrees. You are now at true bottom dead centre for your engine. Reposition the degree disc to 0 BDC (easily done with the magnet holding it) without moving the engine, remove the piston stopper leaving the spark plug hole empty and rotate the engine forward until the pointer shows 0 TDC. You now have true top dead centre on the compression stroke for your engine, making it easy to accurately set your ignition timing at whatever figure 'before top dead centre' it should be.

Now I mentioned earlier that the timing indicator I made for the Cub has two pointers on it. The first is for the degree disc, the second comes into play after the disc is removed to indicate the position of the rotor.

Whilst at TDC, I pop marked the aluminium either side of the rotor and scribed a line between the two which corresponded to where the second pointer is located. Thus doing away with having to use the piston stop method of determining top dead centre in the future.

So there you have it and hopefully the words and picture combined make it easier to understand.

Happy timing!